Donald J. Trump held a surprise news conference shortly before the second presidential debate on Sunday with three women who have accused Bill Clinton of sexual assault or sexual harassment in the past.

A fourth woman who was raped by a man Hillary Clinton defended at trial in 1975 also appeared on the impromptu panel, which Mr. Trump billed as the end of debate preparations and broadcast via Facebook Live.

The four women were Paula Jones, Kathy Shelton, Juanita Broaddrick and Kathleen Willey. Here is a brief guide to the accusations each woman has made against Bill and Hillary Clinton.

Paula Jones

Ms. Jones is a former Arkansas state employee who sued Mr. Clinton for sexual harassment in 1994. She claimed that he exposed himself to her and propositioned her in a hotel room in 1991 when he was governor.

Ms. Jones’s lawsuit led to Mr. Clinton’s impeachment in 1998 because it was during a deposition in that case that he first denied having had sexual relations with the White House intern Monica Lewinsky.

On Sunday, Ms. Jones responded angrily to a reporter who asked Mr. Trump if his star power entitled him to grope women without their consent. “Why don’t you ask Bill Clinton that?” she said. “Why don’t you go ask Bill Clinton that? Why don’t you ask Hillary as well?”

Kathy Shelton

When she was 12, Ms. Shelton alleged that she had been raped by Thomas Alfred Taylor, a 41-year-old man. Mrs. Clinton defended Mr. Taylor in his 1975 trial when she was a professor and a lawyer with the University of Arkansas Law School Legal Aid Clinic.

In a 2014 CNN interview, Mahlon Gibson, the attorney who prosecuted Mr. Taylor, said that Mrs. Clinton had been assigned the case and tried to get out of defending Mr. Taylor, a request that was denied by the judge, Maupin Cummings. “She got appointed to represent this guy,” Mr. Gibson said.

Mrs. Clinton succeeded in getting a plea deal for Mr. Taylor. He was charged with “unlawful fondling of a child under the age of 14” and sentenced to one year in a county jail and four years of probation. In 2014, The Washington Free Beacon published an audio recording in which Mrs. Clinton could be heard chuckling about Mr. Taylor’s case.

On Sunday, Ms. Shelton said that “at 12 years old, Hillary put me through something that you would never put a 12-year-old through” and accused her of “laughing on tape saying she knows they did it.”

Juanita Broaddrick

In 1999, Ms. Broaddrick, a volunteer in Mr. Clinton’s first gubernatorial campaign, accused him of raping her in an Arkansas hotel room in 1978, recanting an affidavit that she had signed the previous year in which she said the episode had not taken place. She was referenced in Ms. Jones’s sexual harassment lawsuit as Jane Doe No. 5.

On Sunday morning, Mr. Trump tweeted a link to an interview Ms. Broaddrick gave to the conservative news website Breitbart in which she repeated her accusations against Mr. Clinton and accused Mrs. Clinton of enabling her husband’s behavior.

Ms. Broaddrick repeated her accusations against the Clintons at Mr. Trump’s hastily arranged news conference on Sunday night, defending him against the maelstrom of criticism caused by the release of a recording on which he could be heard talking about women in vulgar sexual terms.

“I tweeted recently, and Mr. Trump retweeted it, that actions speak louder than words,” Ms. Broaddrick said. “Mr. Trump may have said some bad words, but Bill Clinton raped me and Hillary Clinton threatened me. I don’t think there is any comparison.”

Kathleen Willey

Ms. Willey is a former White House volunteer who claimed Mr. Clinton made a “very forceful” sexual advance against her in his office in November 1993 when she met with him to seek a job.

On Sunday, Ms. Willey said that she was a supporter of Mr. Trump’s campaign and that she believed in his campaign slogan, “Make America Great Again.”

“And I cried when he said that because I think that this is the greatest country in the world,” she said. “I think that we can do anything. I think we can accomplish anything. I think we can bring peace to this world, and I think Donald Trump can lead us to that point.”